Cold-weather concrete: pouring through an El Paso winter

A commercial slab goes down on a clear December afternoon in the Borderplex. The forecast reads 62F, sunny, no wind. The crew places, finishes, and covers up around four in the afternoon feeling good about a mild-weather pour. Then the sun drops behind the Franklins, the desert sky goes clear and cold, and by two in the morning the air is at 29F with the slab radiating heat straight up into a cloudless sky. Nobody planned for that night, and the top inch of that slab is now at risk.

That is the trap of an El Paso winter. The daytime forecast tells you it is mild. The overnight low is what damages concrete. The pour that fails here is almost never the one that got cold during the workday. It is the one that got cold after everyone went home.

The number that matters is 40 degrees

The American Concrete Institute publishes the standard for this work, ACI 306, Guide to Cold Weather Concreting. There is a companion specification, ACI 306.1, that puts the same requirements into contract language with a mandatory-items checklist for owners and engineers. Both point to one trigger.

Cold-weather concreting begins when the ambient temperature falls below 40 degrees Fahrenheit. Below that line, fresh concrete needs active protection: insulated blankets, ground heaters, accelerating admixtures, and in-place temperature monitoring to confirm it has reached strength before the protection comes off.

In El Paso, that 40F line is almost never a problem at noon. It is a nightly problem in December, January, and February, when overnight lows drop into the 30s and into the 20s during a cold snap. The pour has to be planned around the low, not the high.

What cold actually does to fresh concrete

Concrete is a chemical reaction, not a material that simply dries. Cement and water react and harden, and that reaction slows as temperature drops. Cold works against you in two distinct ways.

First, it stalls the reaction. Below 40F, hydration crawls, set time stretches out, and strength gain nearly stops. A slab that would be walkable overnight in mild weather sits soft far longer in the cold, which delays finishing, form removal, and every trade stacked behind the pour.

Second, and far worse, it can freeze the water inside young concrete. If fresh concrete freezes before it has gained enough strength, the water expands into ice and tears the internal paste apart. That damage is permanent. The concrete can lose a large share of its designed strength and the surface can scale, even though the slab looked sound the day it went down.

Why the overnight low is the whole game here

A practitioner rule of thumb puts the danger zone at temperatures below 41F and falling, or below 36F and rising. Unprotected concrete in that window risks cracking and surface scaling.

El Paso hits that zone on clear winter nights even when the day was pleasant. The desert has little humidity and few clouds to hold heat, so a fresh slab radiates warmth into the open sky and its surface can run colder than the air temperature the thermometer reads. That is why we watch the overnight drop closely. A 60F afternoon means nothing if the slab is going to sit at 30F at four in the morning with nothing over it.

The cold-weather plan, start to finish

A real plan controls temperature from the mix truck through the first several nights of curing. This is what we build into a winter pour in El Paso.

• Warm the mix before it arrives. For slabs and sections under 12 inches thick, concrete should reach the site at roughly 50F minimum, and the same target holds for walls, beams, and columns in the 12-to-36-inch range. The plant pre-heats the mix water and, in a real cold snap, warms the aggregate so the truck does not deliver a chilled load.
• Get ice and cold out of the way of the pour. ACI 306 and the ready-mix guidelines require that every surface touching the concrete, subgrade, forms, and reinforcing steel, be free of ice and snow and above freezing before placement. On caliche subgrade after a cold night that can mean thawing and protecting the ground ahead of the truck.
• Design the mix for cold. A chemical accelerator speeds hydration so the slab reaches safe strength faster, shrinking the window it is vulnerable to freezing. Where the concrete will face freeze-thaw exposure, the mix is air-entrained for durability. We place at the lowest practical slump so there is less bleed water sitting on top to freeze.
• Protect the slab through the cold hours. The moment finishing is done, protection goes on: insulated curing blankets, tarps, or insulated mats to trap the heat the concrete generates as it cures. For a hard freeze or a critical pour, a heated enclosure with ground heaters holds the whole placement above the threshold. The goal is to carry the concrete past its vulnerable early age before it ever sees ice.
• Monitor in-place temperature, do not guess. ACI 306 calls for verifying temperature in the concrete itself, not just the air. We place sensors in the slab and watch them through the night so we know it is holding above the line and gaining strength, and so we know when it is safe to pull protection.
• Pull the water, then pull the heat, carefully. Before protection comes off, we remove standing water from the surface 12 to 24 hours ahead, because a saturated top layer is extremely vulnerable to freeze-thaw damage even when the concrete below is sound. Then we let the temperature come down gradually rather than exposing a warm slab to a cold shock, which prevents thermal cracking.

Test cylinders have to be protected too

Cold weather does not only threaten the slab. The test cylinders that prove the concrete met spec are just as sensitive. NRMCA guidance and the standard practice require field cylinders be initially cured at 60 to 80 degrees Fahrenheit for the first 24 to 48 hours, with a minimum-maximum thermometer in the curing box keeping a temperature record.

Leave the cylinders in a cold field box overnight and they can test low even when the in-place slab is fine, which turns into a fight over acceptance nobody should have to have. On a winter job, protecting and heating the cure box is part of the plan, not an afterthought.

The upside nobody expects

Here is the part that surprises owners. Concrete placed in cold weather, protected from freezing and cured for a sufficient length of time, can develop higher ultimate strength and greater durability than the same mix placed in the heat, and it is less prone to thermal cracking.

The catch is entirely in the protection. Cold-weather concrete is not a weaker pour, it is a pour that punishes a missing plan. Get the protection and the monitoring right and the winter slab can outlast the summer one.

Why self-performing concrete matters here

A cold-weather pour fails at the seams between trades. The mix temperature, the subgrade, the placing crew, the blankets and heaters, and the overnight monitoring all have to move as one, with someone accountable for the slab at two in the morning. Split those across a general contractor and three separate subs and the plan gets handed off until nobody owns the coldest hour.

Expert Construction Group self-performs concrete. Our own crews place, finish, protect, and monitor, and we coordinate the mix and timing directly with the plant. One team owns the delivery temperature, the ground prep, the protection through the night, and the call on when to pull it. In a Borderplex winter, that single line of ownership is the difference between a slab that gains strength overnight and one that quietly froze while the forecast said mild.

Send us the pour

If you have a winter slab, foundation, or structural pour coming up in El Paso or Las Cruces, send your plans or your bid set to our estimating team. We will give you an honest read on the schedule, the cold-weather protection it needs, and what a sound winter pour actually costs. You win the work with us or you do not, but you will know what the overnight low can do to a pour before the first truck rolls.